While most US boats were well made, some were quite shoddy (typical of wartime construction). Most U-boats were very well made, though in the end the last U-boats began to suffer. The XXI's, as actually turned out, (not designed) were poorly made and probably had effective dive depths less than a VIIC, due to defective welding. At least that is what the Yanks concluding in examining them post war. Post war US analysis of U-boats turned up a dismaying inferiority in US design and fab to the U-boats in a variety of areas. I tried to touch on the more major issues above.

Overall, I'd say the German boats were better made by a good margin. The caveat is the American engine indirect drive system was much more reliable and failure tolerant, and so the propulsion system was more reliable in the US boats. I also think the US boat DESIGN much better fit the combat need they were used for than the VIIC.